1. Field of the Disclosure
This disclosure relates generally to control valves and, more particularly, to pressure actuators of the pressure-to-close type and to the use of air passages in the upper diaphragm casing.
2. Description of the Prior Art
Control valve pressure actuators of the pressure-to-close type are designed to fail such that the diaphragm of the control valve remains in an open position, at the top of the inside surface of the upper diaphragm casing, in the event of a loss of air pressure. The diaphragm is biased toward the open position by a plurality of springs and air is introduced through a vent, or air port, provided in the top of the upper diaphragm casing at a high pressure to urge the diaphragm to the closed position, away from the upper diaphragm casing.
Due to the high spring force exerted on the diaphragm in the direction toward the upper diaphragm casing, there can be a problem of the diaphragm becoming sealed against the upper casing. As a result, the diaphragm can stick in the upper, i.e. failed-open position. While some have attempted to overcome this problem through the use of additional materials mounted above the diaphragm and/or below the inside surface of the upper diaphragm casing, it would be desirable if this and other problems in pressure-to-close type control valve pressure actuators could be diminished or altogether eliminated without resorting to additional materials, which detrimentally add cost and manufacturing time, and which nevertheless may not completely prevent the diaphragm from sticking in the open position, particularly after long term use, as over time, such materials may tend to degrade.
A pressure-to-close type control valve pressure actuator is provided with a housing including an upper diaphragm casing and a lower diaphragm casing. Within the housing, a diaphragm made of a cloth-reinforced rubber, such as Nitrile, is mounted on a diaphragm plate, which in turn is mounted on one or more springs. The spring or springs serve to bias the diaphragm toward an open position, i.e. toward the top of the inner surface of the upper diaphragm casing, such that the control valve pressure actuator fails with the diaphragm in the open position in the event of a loss of air pressure.
In order to prevent the diaphragm from sealing against the inner surface of the upper diaphragm casing, it is desirable to allow air to circulate in a region between the diaphragm and the inner surface of the upper diaphragm casing at all times, even when the diaphragm is in its highest, i.e. failed, position. In order to allow such air circulation, we have found that channels defining air passages may be imparted to the upper diaphragm casing.
Such air passages not only increase the effective area available for pressurization on the diaphragm, and thereby overcome the problem of the diaphragm sealing against the inner surface of the upper diaphragm casing, but also advantageously provide additional stiffening of the upper diaphragm casing. This additional stiffening enables the control valve pressure actuator to operate at even higher pressures than conventional diaphragm casings before reaching an overpressure situation.
The air passages are preferably imparted to the upper diaphragm casing at the time of stamping of the upper diaphragm casing, but alternatively could be cast into an upper diaphragm casing. It has been found that various quantities and configurations of the air passages are possible and may be selected by the diaphragm casing manufacturer as desired for a particular sized diaphragm casing. For example, while relatively small upper diaphragm casings may lack sufficient surface area to provide many such air passages while still affording sufficient flat surfaces upon which to provide any necessary and/or desired markings, such as model number, control valve specifications, ratings, manufacturing date, and the like, relatively larger diaphragm casings may have sufficient surface area for comparatively more air passages.
The air passages are able to prevent sealing of the diaphragm to the upper diaphragm casing, at least in part, due to the fact that they increase the effective area available for pressurizing the diaphragm. These and other advantages of the air passages for the upper diaphragm casing will become clear from the following Detailed Description of the Preferred Embodiments and the several views of the drawing, in which: